A small group of hooded protesters broke away from a rally of about 10,000 demonstrators and hurled petrol bombs and firecrackers at police guarding parliament. Police responded with teargas dispersing the crowd temporarily.

As the clashes broke out, Prime Minister Alexis Tsipras was delivering a speech in parliament defending proposed legislation which includes pension cuts and tax hikes to be implemented in 2019 and 2020, if adopted later on Thursday.

A vote was expected around midnight local time (2100 GMT).

Tsipras' coalition, which came to power in 2015 promising to end austerity but was forced to sign up to a third bailout later that year, has a slim majority of 153 lawmakers in the 300-seat parliament.

The leftist-led government hopes that legislating the measures, four days before euro zone finance ministers meet in Brussels, will convince its lenders to release a 7.5 billion euro bailout tranche and grant it further debt relief.